Tests of General Relativity, such as polarization measurements and precise tests of high-spin black-holes, require multiple detectors [1]}, [2]}, [3]}.
Moreover, three or more third-generation gravitational-wave detectors are required to localize the source in the sky and to measure the source distance precise enough to confirm the sources at high redshifts [4]}, [5]}.
This suggests that two Cosmic Explorer facilities along with Einstein Telescope are necessary to achieve the key science goals of third-generation gravitational-wave detectors described here.
